It is not uncommon for hydrocarbon wellbores to pass through two or more productive hydrocarbon formations or zones which have different production potentials. Because the production potentials of the different zones are unbalanced, it is undesirable to permit the fluids from different zones to freely co-mingle as the total production of the well may be adversely affected. In many jurisdictions, government regulations also prohibit the free co-mingling of fluid hydrocarbons from different production zones. One traditional method of producing a dual-zone well is therefore to isolate the zones using packers, or the like, and produce the well one zone at a time from the bottom of the bore up until each zone is exhausted. The problem with this method is that the production of the well fluctuates between production peaks for each zone and marginal production as each zone is exhausted. A further problem with this traditional method is that while a lower zone is being produced, hydrocarbon fluids in an upper zone may be irretrievably lost to another well, or the like.
Another traditional method of producing multi-zone wells is to produce each zone through a separate production string. There are several problems with this method. First, wellbores are often drilled without knowledge of the number of production zones that will be encountered. To economize drilling costs, bores are generally drilled and cased with casing of a diameter that will accommodate only one production tubing string. If multiple production zones are encountered in a wellbore of that diameter, multiple string production is impractical or impossible.
Apparatus for producing multiple zone oil and gas wells has been invented, as taught in U.S. Pat. No. 3,746,089 which issued on Jul. 17, 1973 to Vencil and U.S. Pat. No. 3,765,483 which issued on Oct. 16, 1973, also to Vencil. U.S. Pat. No. 3,346,089 teaches an apparatus for producing two or more oil and gas zones by allowing production from at least one zone to drain down to a common chamber where production from the zones is co-mingled and lifted to the earth surface by a single pump. A problem with this arrangement is that if natural pressure of the two zones is unbalanced, that zone will never be produced until the higher pressure zone is exhausted because the higher pressure zone will always overbalance production, leaving the lower pressure zone unproduced.
U.S. Pat. No. 3,765,483 teaches a method and apparatus for producing dual zone oil wells by permitting hydrocarbon fluids from each zone to drain into separate chambers where production from each zone is pumped by separate pumps driven by a common sucker rod string. The outputs from the separate pumps are comingled and conducted through one tubing to the surface. Production from the lower zone is pumped through a passageway that bypasses the upper zone and into the common tubing annulus where it is output for production. The passageway and a check valve through which production from the upper zone is drawn by a second pump are housed in an annular body. The two zones thus produce fluid hydrocarbons independently, and the hydrocarbons are co-mingled in a top of the production tubing above the second pump. The disadvantage of the apparatus is that the annular body is large complex and requires a wellbore of considerable diameter which could as easily accommodate two production strings.